The present invention relates to improvements in an ultrasonic motor driven by a self-excited oscillator circuit and improvements in electronic devices fitted with an ultrasonic motor.
In recent years, ultrasonic motors producing driving force by making use of the piezoelectric effect of a piezoelectric element have attracted attention in the field of micromotors. A simple self-excited oscillator circuit using excitation of a piezoelectric element is known as a driver circuit for such an ultrasonic motor. It has the advantage that any circuit for varying the frequency in response to variations in temperature or external load is unnecessary.
A known example of such a self-excited oscillator circuit has a piezoelectric element excited at a given frequency, an amplifier circuit for amplifying the excitation signal, and a filter circuit for setting the excitation signal to a given frequency (for example, see Japanese Unexamined Patent Publication No. Hei. 08-251952).
A vibrator bonded to the piezoelectric element is bent and oscillated. A frictional force is applied to a moving body pressed against the vibrator, thus rotating the moving body in a given direction.
In the self-excited oscillator circuit described above, however, the circuit constants such as time constant and resistances are set constants and so the rotational speed of the moving body cannot be controlled. For instance, where an ultrasonic motor is applied to a timepiece and the time indicated by the hands of the timepiece is corrected, a long time is necessary with ordinary rotational speeds. Where the motor is applied to a portable tape recorder and rewinding is done, the rotational speed produced during playback is too slow.
Usually, the circuit constants of the self-excited oscillator circuit are set to values midway between the optimum values where the load is large at start and the optimum values in a steady state. Therefore, the torque necessary at start is insufficient. Meanwhile, in a steady state, the speed does not reach the target rotational speed.
The battery voltage of the power supply used for the amplifier circuit and the like varies with use. Consequently, as the voltage of the power supply varies due to aging characteristics, it is necessary to optimize the voltage delivered to the amplifier circuit.